Mains cable coil

[JumbleDuck]

What if the lead is connected or disconnected while on load on the peak of a half cycle?

Makes no difference - at any point in the cycle you still have the same current flowing in opposite direction down conductors very near each other, so they cancel out as near as makes no difference.

It seems hard to believe there would be enough magnetic flux to demagnetise a compass needle.

Indeed. I wouldn't rule it out as impossible, because unless you wind the coil from coax there is a small field produced, but I'd want to rule out other possible causes first.

My earlier post regarding electrolytic corrosion is no myth though. If you wrap a mains lead around damp metal you do get electrolytic corrosion.

Interesting. Corrosion between what and what? And does the current in the lead affect the results?

 

[rogerthebodger]

The wiring of a compass light is a twisted pair to ensure any magnetic field due to current flow is canceled out.

This is also why data comms cables can be twisted pairs that will cancel out any induces current/voltage due to external magnet field effects.

 

[alanw1947]

The wiring of a compass light is a twisted pair to ensure any magnetic field due to current flow is canceled out.

This is also why data comms cables can be twisted pairs that will cancel out any induces current/voltage due to external magnet field effects.

Correct - but that would normally be a DC circuit. The twisting helps cancel the mag field.

 

[rogerthebodger]

Correct - but that would normally be a DC circuit. The twisting helps cancel the mag field.

Same applies to AC as I posted early.

 

[savageseadog]

Interesting. Corrosion between what and what? And does the current in the lead affect the results?

A current is induced in any metal that the cable is wrapped around, an AC current. I know that a winch I had on the boat suffered that way. Some people think that AC currents can't cause corrosion but electrolysis isn't reversible so on one half cycle electrolysis is induced but the other half cycle doesn't reverse it. I'll have to think about the exact way it happens.

 

[crewman]

I often wrap the shore cable round a cleat on the boat and on the pontoon to keep the cable slack if the boat moves. Will that damage the cleat?

 

[savageseadog]

I often wrap the shore cable round a cleat on the boat and on the pontoon to keep the cable slack if the boat moves. Will that damage the cleat?

I can't be sure exactly what the effect would be. A single solid metal object would probably be OK but where the induced current passes through dissimilar metals electrolysis will occur or be accelerated. The cleat will have stainless screws so the aluminium and/or screws will electrolyse at their interface. The current density from a cable will be quite low and will decrease sharply with distance (inverse square or cube at a guess) so we're talking about months or more for significant results. As I said I lost two winches due to this effect, the wrap was in place for a year or two.

 

[luke collins]

I don’t think at the current, voltage being drawn mixed with a couple of turns and being AC any thing would happen. You also have an earth cable in the mix which may act as a partial shield. The cleat will also be isolated assuming it’s a GRP hull

 

[savageseadog]

I don’t think at the current, voltage being drawn mixed with a couple of turns and being AC any thing would happen. You also have an earth cable in the mix which may act as a partial shield. The cleat will also be isolated assuming it’s a GRP hull

The earth would be no shield. I have wondered if rather than electromagnetic induction it could be capacitive coupling, the live is 240V AC, the Neutral zero, so unbalanced. The cleat being isolated is irrelevant, I'm talking about currents circulated in a local piece of metal.